Patent GB-1506018-A (GB)
Title: Electro-acoustic transducer
IPC: H04R1/10
Filed: 1975-02-27
Abstract: 1506018 Moving armature transducers MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 27 Feb 1975 [28 Feb 1974] 8251/75 Heading H4J An electromagnetic transducer, e.g. for a hearing aid earphone, has a D.C. magnetic flux generating section including a permanent magnet 5 (Fig. 2) arranged spaced from and co-acting with one side of a diaphragm 7 and a signal magnetic flux generating section including a coil 9 arranged spaced from and co-acting with the opposite side of the diaphragm 7. The diaphragm 7 is a peripherally clamped film 7-1 carrying a centre armature of magnetic material. The magnet 5 is fixed centrally of a lower magnetic circuit yoke 6 and the coil 9 is fixed to an upper magnetic circuit yoke 8. A sound outlet 11 is provided. The D.C. magnetic flux # 0  and the A.C. magnetic flux # 1  merge together at the upper yoke 8 and the gap above the diaphragm 7 to develop a diaphragm armature driving force at the gap between the diaphragm 7 and the upper polepiece 8-1. The non-magnetic periphery of the diaphragm 7 inhibits D.C. magnetic flux passing to the periphery of the upper magnetic polepiece 8, thereby preventing magnetic saturation of the diaphragm. The assembly has an outer casing 10 of magnetic material confining the fluxes # 0  and # 1  and inhibiting interference by external magnetic fields. In an alternative arrangement (Fig. 6) a part of the D.C. magnetic flux # 0  from magnet 5 passes along a polepiece 12 and pole plate 13 through the lower magnetic circuit yoke 6. By selection of the size of plate 13 the dimension of gaps 50 and 51 may be minimized without saturating the armature 7-2, the fluxes # 0  and # 1  being large. Laminar air flow in the gap 50 is attained. If desired, a fixing ring (16) Fig. 9 (not shown) of non-magnetic material is introduced between the diaphragm 7-1 and the upper yoke 8, the magnetic flux path between the two yokes being completed through the casing 10 of magnetic material. In yet another arrangement two non-magnetic fixing rings (16, 16') Fig. 12 (not shown) hold the diaphragm 7 between them. Leakage reluctance between the magnetic armature 7-2 and the yokes 8 and 6 is increased. Some practical embodiments are illustrated (Figs. 13, 14, not shown). Fig. 15 shows a polepiece 213 fixed to the permanent magnet 212 and formed to concentrate the D.C. magnetic flux primarily at the periphery. An upper magnetic pole 217 allows flux to pass from the magnet 212 through the armature 215. A.C. signal magnetic flux passes from pole 218 through armature 215 to upper magnetic pole 217. Thus the fluxes # AC  and # DC  superimpose on each other in the gap between the armature 215 and upper magnetic pole 217 (see Fig. 16, not shown). In Fig. 19 an A.C. signal flux # AC  from a signal applied to terminal 512 follows the dotted line path shown. Part of the D.C. flux # DC  passes through the armature 514 to a step 505 of the lower magnetic pole 502 and back to the other pole of the permanent magnet 501. A further portion of the D.C.flux # DC  passes through the magnetic pole 504 to the lower magnetic yoke and back to the other pole of the permanent magnet 501. Driving forces proportional to the signal are generated between the upper magnetic pole 510 and the armature 514 and at the gap between the armature 514 and the step 505. The drive forces in the two regions act in opposite directions and the diaphragm is operated in push-pull mode. Instead of the step 505 a ring (505') (Fig. 20, not shown) of magnetic material may be provided in the plane of the magnetic pole-plate 504. This modification reduces the acoustic chamber stiffness presented by the upper air chamber and improves sensitivity.